1. Technical Field
The invention relates to circuit fabrication. In particular, the invention relates to lithography used to define features during printed circuit and semiconductor fabrication.
2. Description of Related Art
For much of the history of modern electronics, optical lithography has been the principal means of patterning substrates in the fabrication of printed circuits, semiconductors, and related structures. In optical lithography, a photoresist is deposited onto a surface of a substrate being patterned. A source of radiation (e.g., ultraviolet light, electron beam, etc.) is then employed to differentially expose the photoresist and impart a pattern thereto. For example, the photoresist may be exposed to ultraviolet light projected through a photomask such that a pattern of the photomask is transferred to the photoresist. The photoresist bearing the transferred pattern forms a pattern mask that protects specific portions of the substrate that are covered by the pattern mask. Various techniques such as pattern etching and pattern lift-off are then employed to transfer the pattern of the photoresist pattern mask to a surface of the substrate.
Relatively recently, other methods of creating a pattern mask for substrate patterning have been proposed and, in some cases, have begun to gain acceptance as viable alternatives to photolithography. Among these techniques are imprint lithography and digital resist printing. In imprint lithography, a pattern mold is employed to mechanically imprint a pattern into a relatively soft resist material on a surface of the substrate. The imprinted soft resist material is then employed as a mask in a manner similar to the photoresist pattern mask.
In digital resist printing, a pattern mask is printed directly onto the substrate using a digital printer or equivalent. For example, an inkjet printer may be employed to print a liquid resist onto a substrate in a manner similar to that used to print an image onto a piece of paper. The inkjet printer essentially applies the liquid resist as a series of directed droplets that combine to form the pattern mask. In another example, a laser printer is employed to directly print the pattern mask onto the substrate. The laser printer employs a dry toner comprising a fine powder that is electrostatically guided and imaged onto the substrate under digital control of the printer. After being imaged, particles of the dry toner are fused together on the substrate to form the pattern mask. Either of the inkjet printer-applied liquid resist or the laser printer-applied dry toner produces a pattern mask that may be employed in a manner similar to the photoresist pattern mask to pattern the substrate.
Advantageously, digital resist printing does not depend on the creation and use of either a photomask or a mold. As such, the mask pattern can be readily changed or adjusted during substrate patterning. However, inkjet-based resist printing is relatively slow. Thus, inkjet-based resist printing may be of limited use, especially for high speed and/or high volume printed circuit production (e.g., roll-to-roll processing). The use of laser printers for direct printing of a dry toner-based pattern mask can support high speed, high volume production. However, the dry toner employed by laser printers severely limits an ultimate resolution of the pattern and further suffers from problems of line edge fidelity and unwanted background printing caused by powder dispersion during printing.